Main brake cylinder for a hydraulic vehicle brake system and method for operating same

Abstract

A main brake cylinder for a hydraulic vehicle brake system of a motor vehicle is disclosed. The main brake cylinder includes an electric motor that is operated as a generator for decelerating the motor vehicle. For service braking, pressure is applied to a brake circuit by the main brake cylinder and to another brake circuit by a hydraulic pump. In order to reduce a pedal force, a first piston has a smaller piston surface and a second piston only moves if a greater brake fluid volume is required. A preloaded hydraulic accumulator limits an increase in pressure.

The invention claimed is: 
     
       1. A hydraulic vehicle brake system of a motor vehicle comprising:
 a master brake cylinder including a first piston, a second piston, and a carrier device; 
 a first brake circuit; 
 an electric machine configured to be operated as a generator for decelerating the motor vehicle; and 
 a hydro-accumulator configured to fluidly communicate with the master brake cylinder, 
 wherein the carrier device is configured such that after movement of the first piston by a prespecified piston travel, the carrier device carries the second piston with the first piston, 
 wherein a combined piston area of the first and second pistons active for displacement of brake fluid from the master brake cylinder is greater than a piston surface of the first piston active for displacement of brake fluid from the master brake cylinder, 
 wherein the hydro-accumulator is configured to have a pretension such that the hydro-accumulator only receives brake fluid from the master brake cylinder when a master brake cylinder pressure is greater than a maximum design pressure of the vehicle brake system, and 
 wherein the hydro-accumulator is configured such that the maximum design pressure is equal to or greater than an assumed maximum muscle force exerted on a brake pedal to move both the first and second pistons such that the hydro-accumulator only receives brake fluid from the master brake cylinder when only the first piston of the master brake cylinder moves. 
 
     
     
       2. The hydraulic vehicle brake system as claimed in  claim 1 , wherein the first piston lies within the second piston. 
     
     
       3. The hydraulic vehicle brake system as claimed in  claim 1 , further comprising a brake fluid storage reservoir, wherein:
 when the master brake cylinder is not activated, a pressure chamber of the master brake cylinder communicates with the brake fluid storage reservoir, and 
 by a short movement of the first piston, the pressure chamber is isolated from the brake fluid storage reservoir. 
 
     
     
       4. The hydraulic vehicle brake system as claimed in  claim 1 , wherein the master brake cylinder is a tandem or multicircuit master brake cylinder for a dual-circuit vehicle brake system. 
     
     
       5. A method for operation of a hydraulic vehicle brake system of a motor vehicle, which comprises (i) a master brake cylinder having a first piston, a second piston, a third piston, and a carrier device, which after movement of a first piston over a pre-specified piston travel carries the second piston with the first piston, the master brake cylinder including a first chamber defined between (a) the first and second pistons and (b) the third piston, and a second chamber defined by the third piston remote from the first and second pistons, the first chamber being hydraulically connected to a first brake circuit and the second chamber being hydraulically connected to a second brake circuit, wherein a piston surface area of both pistons active for displacement of brake fluid from the master brake cylinder is greater than a piston surface of the first piston active for displacement of brake fluid from the master brake cylinder, the method comprising:
 during a service braking operation, closing an isolating valve, which connects the second brake circuit of the vehicle brake system to the second chamber of the master brake cylinder; 
 during the service braking operation, operating an electric machine connected to the second brake circuit as a generator to decelerate the motor vehicle; 
 during the service braking operation, generating a brake pressure with a hydraulic pump; 
 regulating a wheel brake pressure in at least one wheel brake of the vehicle brake system depending on activation of the master brake cylinder and on a deceleration effect of the electric machine; and 
 in response to a pressure in the first chamber exceeding a maximum design pressure, receiving fluid in a hydro-accumulator, which is positioned between the first chamber and the first brake circuit and is pretensioned to only receive fluid if the maximum design pressure is exceeded, to prevent damage to components in the first brake circuit. 
 
     
     
       6. The method as claimed in  claim 5 , the regulation of the wheel brake pressure further comprising operating at least one wheel brake pressure control valve to regulate the wheel brake pressure in the at least one wheel brake of the vehicle brake system. 
     
     
       7. The method as claimed in  claim 5 , wherein the master brake cylinder is a tandem or multicircuit master brake cylinder and the vehicle brake system comprises at least one further brake circuit which in service braking remains connected with the master brake cylinder. 
     
     
       8. The method as claimed in  claim 5 , wherein the hydro-accumulator is configured such that the maximum design pressure is equal to or greater than an assumed maximum muscle force exerted on a brake pedal to move both the first and second pistons such that the hydro-accumulator only receives brake fluid from the master brake cylinder when only the first piston of the master brake cylinder moves. 
     
     
       9. A hydraulic vehicle brake system of a motor vehicle comprising:
 a master brake cylinder including a first piston, a second piston, and a carrier device; 
 a first brake circuit; 
 a second brake circuit; 
 an electric machine connected to the second brake circuit and configured to be operated as a generator for decelerating the motor vehicle; and 
 a hydro-accumulator hydraulically connected between the first brake circuit and the master brake cylinder, 
 wherein the carrier device is configured such that after movement of the first piston by a prespecified piston travel, the carrier device carries the second piston with the first piston, 
 wherein a combined piston area of the first and second pistons active for displacement of brake fluid from the master brake cylinder is greater than a piston surface of the first piston active for displacement of brake fluid from the master brake cylinder, and 
 wherein the hydro-accumulator is configured to have a pretension such that the hydro-accumulator only receives brake fluid from the master brake cylinder when a master brake cylinder pressure is greater than a maximum design pressure of the vehicle brake system. 
 
     
     
       10. The hydraulic vehicle brake system as claimed in  claim 9 , wherein:
 the master brake cylinder includes a third piston, 
 a first chamber is defined in the master brake cylinder between (i) the first and second pistons and (ii) the third piston, the first chamber being hydraulically connected to the hydro-accumulator and the first brake circuit; and 
 a second chamber is defined by the third piston remote from the first and second pistons and is hydraulically connected to the second brake circuit. 
 
     
     
       11. The hydraulic vehicle brake system as claimed in  claim 10 , further comprising:
 an isolating valve disposed in the second brake circuit configured to isolate the remainder of the second brake circuit from the second chamber, 
 wherein the isolating valve is closed during a service braking operation to isolate the second brake circuit from the master cylinder. 
 
     
     
       12. The hydraulic vehicle brake system as claimed in  claim 9 , wherein the first piston lies within the second piston. 
     
     
       13. The hydraulic vehicle brake system as claimed in  claim 9 , further comprising a brake fluid storage reservoir, wherein:
 when the master brake cylinder is not activated, a pressure chamber of the master brake cylinder communicates with the brake fluid storage reservoir, and 
 by a short movement of the first piston, the pressure chamber is isolated from the brake fluid storage reservoir.